Shell mold composition

ABSTRACT

Composition for making shell molds for precision casting of metals containing silica, aqueous collodial silica and graphite in sufficient amounts to control certain properties of the finished mold.

United States Patent Sulinski [4 1 Apr. 18, 1972 [54] SHELL MOLDCOMPOSITION 72 lnventor: Harry v. Sulinski, Philadelphia, Pa. [56]Refmms CM [73] Assignee: The United States of America as UNITED STATESPATENTS represented by the Secretary of the y 2,948,032 8/1960 Reuter 106138.3 x [22] Filed: Oct 14, 1970 3,396,935 8/1968 Synder 106/389 X[21] App]. No.: 80,793 Primary Examiner-Lorenzo B. Hayes Attorney-HarryM. Saragovitz, Edward J. Kelly, Herbert Berl Related U.S. ApplicationData and Sheldon Kanars -[63] Continuation-in-part of Ser. No. 8,697,Feb. 4, 1970,

abandoned, which is a continuation-in-part of Ser. No. [57] ABSTRACT780,605, 1399- 2, 1968 abandoned- Composition for makingshell molds forprecision casting of I metals containing silica, aqueous collodialsilica and graphite "106/383! 106/3835, 106/389 in sufficient amounts tocontrol certain properties of the [51] Int. Cl ..B28b 7/34 finished mold[58] Field of Search ..l06/38.2-38.9,

10 Claims, No Drawings SHELL MOLD COMPOSITION This application is acontinuation-in-part of my co-pending patent application, Ser. No.8,697, filed Feb. 4, 1970, now abandoned, entitled Method and Means forFabricating Complex Metal Powder Parts, which, in turn, is acontinuation-in-part of application Ser. No. 780,605, filed Dec. 2,1968, now abandoned, entitled A Process for Fabricating Complex MetalPowder Parts and relates to powder metallurgy and more particularlyconcerns the fabrication of porous metal powder parts of complex shapefrom improved ceramic shell molds.

The invention described herein may be manufactured, used, and licensedby or for the Government for governmental purposes without the paymentto me of any royalty thereon.

Powder metallurgy is a versatile method for producing many materials forvarious requirements in the form of finished parts or semi-finishedforms. In the area of porous materials, however, an improvedmanufacturing technique is needed for forming complex shaped porousparts which cannot be conventionally cold pressed, i.e., parts whichcontain undercuts, re-entrant angles and holes normal to the axis ofcompaction. The methods commonly used in powder metallurgy for producingsuch shapes are slip casting and the process of sintering metal powderin machined metal or graphite molds. Both of these methods, however, arecumbersome and expensive, have shape limitations and are not readilyadaptable to mass production. Those parts which from a bridge in themold or contain cores present difficulty in withdrawal and tend to tearas the part shrinks in the mold.

It is therefore an object of this invention to substantially overcome orat least minimize the disadvantages aforementioned.

Another object of the invention is to provide means for fabricatingpowder metallurgy parts using improved ceramic shell molds.

Still another object of the invention is to provide means foreconomically fabricating high quality complex metal powder parts havinga broad range of shapes and sizes.

Other objects and advantages of the invention will in part be obviousand in part appear hereinafter in the following detailed description.

Very briefly, the invention involves filling an expandable ceramic shellmold with metal powder and sintering the powder in situ.

More specifically, and in accordance with the objects aforementioned,ceramic shell molds, heretofore employed in the foundry industry formaking castings, have been improved so slurry with stuccoed grain areused. Metal powder shapes comprising bars, rods, discs, and the like canreadily be formed using the abovedescribed conventional ceramic shellmolds. More complex parts, however, having cores and recesses therein,exhibit distortion and develop cracks due to restraint offered by themold during metal powder shrinkage. Difficulties are also encountered inremoving the mold material from recessed portions of complex parts. Theabove infirmities are believed to be related to the strength of themold. The strength of the mold could reduce by the simple expedient ofreducing the number of coatings. It was determined that at least 3coatings were necessary in order for a mold to withstand the dewaxingoperation. When complex powder metal parts were fabricated from copper,bronze, or stainless steel powders, the pieces were distorted andcracked due to mold restraint of metal powder shrinkage.

The slurry abovedescribed comprises fused silica, liquid colloidalsilica, and a wetting agent. By modifying this standard slurry inaccordance with one aspect of this invention, excellent controlledcollapsibility characteristics of the mold are achieved. Controlledcollapsibility of the mold means that it will predeterminedly break downto thus ofier no restraint to the shrinkage of the metal powder duringsintering, and permitting crack-free complex parts to be producedtherefrom. This controlled collapsibility characteristic of the mold isachieved by the addition of graphite and water to the slurry mixture offused and colloidal silica, and wetting agent, if necessary. Thestrength of the mold decreases with increasing amounts of graphite andattendant water. The geometry of the parts to be fabricated dictates theamount of graphite needed. Those parts which form a bridge in the moldor contain cores and the like require about 6 to 9 percent graphite oreven up to about 10 percent graphite. Simple shapes require less thanabout 6 percent graphite or, if very simple, such as rods, bars, and thelike, little or no graphite will be needed. Thus, my inventive conceptcomprises the addition of about 1.13 to 10 weight percent of graphite tothe slurry, and water, if necessary, to control the collapsibility ofthe mold.

The following examples illustrate typical amounts of graphite needed forfabricating various powder metallurgy parts. Included in each exampleare typical respective refractory slurry compositions:

EXAMPLE I Metal Powder Part: Rod, Bar, Plate or Disc Amount of Graphite:1.13 wt. percent Refractory Slurry Compositions:

Stucco grain mesh fused silica 20 mesh fused silica I Slurry No. 1contains -200 mesh fused silica. 2 Slurry No. 2 contains mesh fusedsilica. Triethanolamirie salt of dodecyl benzene sulfonic acid.

4 Drops.

as to be most useful in making complex powder metal parts, when myprocesses are followed, to be described hereinafter.

The procedure for making the prior art ceramic shell molds is not new.Ordinarily, four to seven coatings of refractory 75 As aforediscussed,for very simple shapes, such as rods, bars, and the like, it may beadvantageous to use no graphite at all, or in amounts even less thanabout 1.13 weight percent, depending upon the nature of the materialsbeing used.

EXAMPLE ll Metal Powder Part: Pulley Wheel Stucco Grain: 80 mesh FusedSilica No. 2 Slurr Amount of Graphite: 4 wt. y Refractory Compositions:Fused Silica, l mesh 1 pound Slurry No. 1 Slurry No 2 Material AmountWt. percent Amount Wt. pcrccm l nsv1lsilirn'- 2 151141118 130,157 151gllls 51.52 (olloitlal .silicn 17! cc Jill. .25 261) cu. 35.41]\Vvttingugollt. iltlrops... 3 (imphilc, l25 mosh. 4.01) 35.2. 4.00\Vnlcl' .02 0b.... 5. 08 80.02 cc fl. ()8 Slurry viscosity. 2,100 to2,500 cps. at. 75 to 85 F. 350 to 500 cps. at. 70 to 75 F. Stucco grain80 mesh fused silica -.2t)1nush fused Silica l Slurry N0. 1 contains 200mesh fused silica.

2 Slurry No. .2 contains 100 mesh fused silica.

3 '1riot.hanolannnc salt of dodccyl licnezene sullonic acid.

l Drops.

M LE I Colloidal Silica, 260 cc 20 Graphite, -325 mesh M t lPowde PartHollow c linder e a r y Same as in No. l Slurry above Amount OfGraphite: 9 Wi As required to control mold strength Refractory-S yCOmPOSIIIOHSI Wt. percent water= (3.27 X wt. percent graphite) 4 SlurryNo. 1 Slurry No. 2

Material Amount Wt. percent Amount Wt. percent Fused silica 1 454 gms51.43 38. 86 Colloidal silica. 172 cc. 25. 64 26. 71 Wetting agent 3 5drops... 4 5 Graphite, 3.!5n1csli 70.45 grits 9.00 \Vatcr. 122..l8cc.13.93 25.43 Slurry viscosity. 2,100 to 2,500 cps. at 75 to 85 F. F.Stucco grain 80 mesh fused silica mesh fused silica Slurry No. 1contains 2l)tl mosh fusod silica.

2 Slurry No. 2 contains 1lill mosh fused silica.

-' 'lrii-tlinnolalnino salt. of dodocyl lmnzono sulfonic acid. Drops.

No. l Slurry: wt. percent water (1.77 X wt. percent Graphite) 2 No. 2Slurry: wt. percent Water (3.27 X wt. percent Graphite) 4 TABLE 1REFRACTORY SLURRY DATA No. l Slurry Amount 1 pound 172 cc Material FusedSilica, 200 mesh Colloidal Silica Wetting Agent Graphite, 325 mesh WaterColloidal silica, aqueous based, having a minimum specific gravity of1.200 at oilF. and containing approximately 30% by weight OI'SiO Wettingagent, an alkyl aryl sulfonate, anionic surface-active, suitably atriethanolamine salt ofdodecyl benzene sulfonic acid.

Afew drops(2to5) As required to control mold strength Wt. percent watert 1.77 x wt. percent graphite) 2 Viscosity ofthe Slurry: 2.l00 to 2,500cps at 7 F.

Viscosity of the Slurry: 350 to 500 cps at 70 75F. Stucco Grain: 20 MeshFused Silica To lend more precision to the quantity of wetting agentrequired, if about pounds of fused silica is present with about 60pounds colloidal silica, then about 1 to 2 teaspoons of wetting agentwill suffice ordinarily. The rule is not to use more wetting agent thannecessary to make the slurry wet the pattern surfaces.

No wetting agent is needed with No. 2 slurry but the addition of only ateaspoon to about 250 pounds of total silica will make a smootherslurry.

In calculating the weight of water needed for the amount of graphitepresent, it is apparent that if only about 1% graphite is present, thenfrom the formulas presented above, no water need be added. In the caseof No. 1 slurry, any amount of graphite present which is below about1.13 percent will require no water. Similarly, in slurry No. 2, if thetotal amount of graphite present is below about 1.22 percent of thetotal weight, no water need be added. The upper limit of graphite forvery complex parts will be about 10 percent of the total weight of theslurry.

The ratios of fused silica to liquid colloidal silica may also varysomewhat from those presented in the above table in order that theviscosity requirements above listed will be met.

Ceramic shell molds may be suitably dewaxed by flashdewaxing the moldsin a hot furnace. Alternatively, the molds may be dewaxed using anautoclave, a molten bath of fusible alloy, hot refractory powder, hotmetal shot or a solvent vapor such as trichlorethylene vapor.

A typical procedure for making my dual strength ceramic shell mold, Le,a mold having high green strength and a low fired strength, is afollows:

1. A wax pattern assembly is washed in acetone to remove any residualdie lubricant therefrom.

2. The pattern assembly in dipped into No. l slurry, drained, and whilestill wet, stuccoed with coarse grain using a fluidized bed of 80 meshfused silica powder to form a first coating of the mold.

3. The mold is dried.

4. The mold is dipped into water, and while still wet, dipped into No. lslurry, drained and stuccoed with silica powder as described in Step 2.

5. The mold is dried.

6. The mold is dipped into water, and while still wet, dipped into No. 2slurry, drained and stuccoed with minus mesh fused silica powder.

7. The mold is then thoroughly dried.

8. Steps 6 and 7 may be repeated, if desirable.

9. The mold is dewaxed.

10. The mold is fired at l,400-1,600 F in an oxidizing atmosphere toburn off the graphite and any wax or carbon residue remaining from thedewaxing operation. The molds thus prepared may be stored indefinitelyand used as needed.

Stainless steel, copper, bronze, and nickel-coated tungsten 4 powders,among others, have been used successfully with my invention. Thesepowders were spherically-shaped of high tap density. Their tap densitywas at least 55 percent of theoretical.

Complex porous metal powder parts were successfully fabricated using thefollowing sintering schedules:

TABLE II.-COMPLEX METAL POWDER PART FABRICAT- ING INFORMATION innitrogen and for minutes at 1,400 F. in a dissociated ammoniaatmosphere. Nickel-coated tungsten (0.25% Sintered 16 hours at 2,500 F.in a dis- N i). sociate-d ammonia atmosphere.

The sequence of operations for fabricating complex metal powder parts aspracticed in this invention is as follows:

1. The ceramic shell mold is manufactured to the desired strength levelin order to obtain the desired collapsibility.

2. The mold and metal powder are conditioned by drying in an oven atabout 250 F in order to drive-off absorbed moisture.

3. The mold is filled with metal powder using a vibrator to insurecomplete filling and dense packing.

4. The metal powder in the mold is presintered to a coherent metalpowder part with a minimum amount of mold restraint of metal powdershrinkage. This treatment alone, in many cases, may be sufficient toimpart the final desired properties, e.g., fabrication of filters.

5. The mold material is removed from the metal part.

6. The metal powder part is sintered to the final desired propertylevel.

It is apparent from the foregoing description that complex partscontaining undercuts, re-entrant angles and cores may readily befabricated from powdered metals by using ceramic shell molds having amost desirable characteristic of controlled collapsibility. Further, thelimitations on shape and size which existed has been substantiallyovercome.

lclaim:

1. ln a slurry for a ceramic shell mold, said slurry including fusedsilica and aqueous colloidal silica;

the improvement which comprises a quantity of graphite in said slurry,said graphite being present in an amount of from about 1.13 to about 10percent by weight of said slurry, said amount being sufficient toprovide dual strength and controlled collapsibility characteristics tosaid mold.

2. The slurry as described in claim 1 wherein said fused silica is about1OO mesh in grain size and is present in said slurry with colloidalsilica in a ratio of about 1 pound to about 260 cc respectively, whereinsaid graphite is about -325 mesh in size, and wherein water is presentin said slurry and the percent graphite exceeds about 1.22 percent ofthe weight of the said slurry.

3. The slurry as described in claim 1 wherein the graphite exceeds about1.22 percent of the weight of the said slurry, and wherein water ispresent in an amount of about (3.27 X wt. percent graphite) minus 4.

4. The slurry as described in claim 3 wherein the viscosity of saidslurry is about 350 to 500 centipoises at 70 to F.

5. The slurry as described in claim 1 further characterized by minoradditions of an anionic surface-active wetting agent thereto, saidwetting agent being an alkyl aryl sulfonate.

6. The slurry of claim 5 wherein said wetting agent is triethanolaminesalt of dodecyl benzene sulfonic acid.

7. The slurry as described in claim 5 wherein said fused silica is about200 mesh in grain size and is present in said slurry with colloidalsilica in a ratio of about 1 pound to about 172 cc respectively, whereinsaid graphite is about 325 mesh in size, wherein water is present insaid slurry and the percent graphite exceeds about 1.13 percent of theweight of said slurry, and wherein said wetting agent is present in aquantity of two to five drops.

8. The slurry as described in claim 7 wherein said wetting agent is atriethanolamine salt of dodecyl benzene sulfonic acid.

9. The slurry as described in claim 1 wherein the graphite exceeds about1.13 percent of the weight of said slurry, and wherein water is presentin an amount of about (1.77 X wt. percent graphite) minus 2.

10. The slurry as described in claim 9 wherein the viscosity of saidslurry is about 2,100 to 2,500 centipoises at 75 to F.

2. The slurry as described in claim 1 wherein said fused silica is about-100 mesh in grain size and is present in said slurry with colloidalsilica in a ratio of about 1 pound to about 260 cc respectively, whereinsaid graphite is about -325 mesh in size, and wherein water is presentin said slurry and the percent graphite exceeds about 1.22 percent ofthe weight of the said slurry.
 3. The slurry as described in claim 1wherein the graphite exceeds about 1.22 percent of the weight of thesaid slurry, and wherein water is present in an amount of about (3.27 Xwt. percent graphite) minus
 4. 4. The slurry as described in claim 3wherein the viscosity of said slurry is about 350 to 500 centipoises at70* to 75* F.
 5. The slurry as described in claim 1 furthercharacterized by minor additions of an anionic surface-active wettingagent thereto, said wetting agent being an alkyl aryl sulfonate.
 6. Theslurry of claim 5 wherein said wetting agent is triethanolamine salt ofdodecyl benzene sulfonic acid.
 7. The slurry as described in claim 5wherein said fused silica is about -200 mesh in grain size and ispresent in said slurry with colloidal silica in a ratio of about 1 poundto about 172 cc respectively, wherein said graphite is about -325 meshin size, wherein water is present in said slurry and the percentgraphite exceeds about 1.13 percent of the weight of said slurry, andwherein said wetting agent is present in a quantity of two to fivedrops.
 8. The slurry as described in claim 7 wherein said wetting agentis a triethanolamine salt of dodecyl benzene sulfonic acid.
 9. Theslurry as described in claim 1 wherein the graphite exceeds about 1.13percent of the weight of said slurry, and wherein water is present in anamount of about (1.77 X wt. perceNt graphite) minus
 2. 10. The slurry asdescribed in claim 9 wherein the viscosity of said slurry is about 2,100to 2,500 centipoises at 75* to 85* F.